1. Field of the Invention
The present invention relates to the synthesis of a non-halogen flame retardant oligomer containing highly pendant phosphorus moities. In particular, it relates to a process for preparing the flame retardant oligomer including first forming a polyester oligomer, and then grafting a phosphorus-containing compound onto the oligomer through an addition reaction.
2. Description of the Prior Art
Flame retardant and heat resistant polymeric materials can be obtained by polymer modification. This modification method involves reacting the flame retardant monomers onto the polymeric backbones by copolymerization. Because the flame retardant monomers are chemically bonded to the polymeric backbones, they will not migrate to the surface of the polymeric resins during processing such as extruding or injecting molding, and thus physical properties and flame retardancy are not affected. Moreover, when the thus obtained polymeric materials are woven into fabrics or are formed into nonwoven fabrics, they display good washing fastness and permanent flame retardancy. However, the process of copolymerization is complicated.
To obtain flame retardancy, an easier way is to coat a layer of a flame retardant on fabricated articles of polymeric resins. This method has the advantage of easy treatment. However, this method also has the disadvantages that only the surface of the fabricated articles are flame retardant and when they are formed into fabrics, the flame retardancy will gradually diminish with repeated washings. That is, the washing fastness of the thus treated fabrics is unsatisfactory.
Blending flame retardants with polymeric resins is the most convenient way to obtain flame retardant polymeric materials. However, organic flame retardants usually can not withstand processing at an elevated temperature, and therefore a lot of organic flame retardants in the blend is lost due to decomposition. In addition, poor compatibility between polymers and flame retardants often cause the migration of flame retardants when the blend is processed or used.
Recently, phosphorus-containing and high molecular weight flame retardants have been developed to replace conventional halogen-containing flame retardants to meet the requirements of low toxicity, low smoke and low migration in view of environment protection and public safety. For example, CR733 of Daihachi Chemical Industry Co., Ltd, Japan, and RDP and Fyrol of AKZO Co., Ltd. are phosphate flame retardants having a molecular weight of less than 2000. These kinds of phosphorus-containing flame retardants are usually synthesized by first reacting phosphorus-containing compounds with unsaturated diacids at a temperature of 150.degree. C. for about 4 hours to form phosphorus-containing comonomers, and then reacting the phosphorus-containing monomers with saturated dicarboxyl acids and diols at an elevated temperature for example, 180.degree.-200.degree. C. to form the final products. The second step of this synthesis usually lasts for about 9-11 hours, which means a total synthesis time of 13-15 hours. Because the phosphorus-containing comonomers are subjected a lengthened period of reaction, they decompose, and then migrate during the reaction. Thus the final products have poor heat resistance and low phosphorus content, and are deeply colored, usually brown in color,.